Worries about thawing permafrost, and the carbon emissions it produces as it warms, have become a standard in conversations about climate change — but scientists are quick to point out that there’s a lot they still don’t understand. And one of the biggest questions has to do with some of the tiniest inhabitants of the Arctic.
Microorganisms in the Arctic soil are a critical component of permafrost’s role in the carbon cycle. These little guys munch on organic matter in the ground — the process we know as “decomposition” — and produce carbon byproducts, such as carbon dioxide and methane, that can then escape into the atmosphere.
In general, scientists know that warming causes permafrost to thaw, which results in more active microbes and greater carbon emissions. But how exactly these microbial communities are changing in response to the warming soil has been poorly understood. “We use models to predict how ecosystems will respond to temperature increase or [carbon dioxide] increase,” said Jizhong Zhou, director of the Institute for Environmental Genomics at the University of Oklahoma. “But we have little knowledge about how microbial communities will respond to this.”
Now Zhou and a group of colleagues from universities in both the U.S. and China have published a new study, out today in the journal Nature Climate Change, which attempts to shed some light on the specific ways microbe communities respond to hikes in their soil temperature, and what that might mean for the amount of carbon they produce. It’s an important question given that many experts fear permafrost could become a significant source of carbon feedback in the future. The warmer it gets, the more carbon emissions the thawing permafrost produces, which accelerates atmospheric warming and causes even more thawing in a kind of a vicious cycle.
“We definitively need more knowledge about responses in the microbial community, and especially in the real situation, the field, that is addressed in this study,” said Mette Marianne Svenning, a professor of Arctic and marine biology at the Arctic University of Norway, who was not involved with the new paper.
For this study, the researchers set up a field experiment on the Alaskan tundra by sectioning off a series of plots on the ground. They were able to manipulate the soil temperature in some of these plots by adding snow in the winter, which insulates and warms the ground. (In the spring, they removed the extra snow to prevent excess meltwater from running into the ground). After one and a half years, they collected samples from all the plots and compared the microbial communities in the warmed plots with the unaltered ones. They also made observations about plant productivity, decomposition rates and net carbon losses from the various plots.
It turns out that warming has a big effect on microbial communities — right down to their genetic makeup. Microbe communities in the warmed plots showed a significant increase in certain genes associated with their ability to decompose carbon. This finding suggests that more warming could cause an increase in the amount of emissions produced, enhancing the microbes’ role in the warming feedback system. And in the warmed study plots, ecosystem respiration — essentially, the amount of carbon dioxide produced by the system — increased by up to 38 percent. The study’s genetic findings also suggested that the microbes could be producing greater amounts of other types of emissions, such as methane, added Mengting Yuan, a research assistant at the University of Oklahoma’s Institute for Environmental Genomics and another of the paper’s coauthors.
On the other hand, warming also seemed to increase genes involved in the process of soil nutrient cycling, which helps convert nitrogen into usable forms for plants. This is important to note because more nitrogen can allow for the growth of more plants, which act as carbon sinks and could potentially offset some of the soil’s carbon losses. Indeed, in the study plots, measures of plant productivity showed an increase of about 30 percent.
In some places, plants may indeed be able to offset carbon emissions — a possibility that could be important for scientists to factor in when producing models of the future Arctic. But in this particular study, that was not the case. The researchers observed that the warmed plots exhibited significantly higher net carbon losses than the control plots, suggesting that the increased plant production was not enough to offset the increased decomposition.
This may be worrying in and of itself — but the authors are also careful to note that another significant aspect of the study is how quickly the microbial communities were altered under warming conditions. “We demonstrated that in one and a half years you see dramatic changes,” said Zhou — a finding that highlights the ecosystem’s high sensitivity to climate warming.
“Overall, whether the tundra soil acts as a [carbon] source or sink depends on plant and microbial responses to climate warming,” the authors write in the paper. “Our results indicate that the soil [carbon] is highly vulnerable to climate warming and this vulnerability is determined by a set of complex microbial feedbacks to the temperature increase.”
One point to note is that long-term trends show that winter temperatures in the Arctic are changing more dramatically than summer temperatures as the climate warms. So one suggestion for future research is to look more closely at some of these processes specifically in the wintertime, said Vladimir Romanovsky, a professor of geophysics and permafrost researcher at the University of Alaska Fairbanks, who was not involved with the study. These days, soil is remaining unfrozen for longer and longer periods of time during the winter, he pointed out — so many of the fundamental changes taking place in microbial communities may actually be happening during the winter.
Svenning, of the Arctic University of Norway, also suggested the importance of more targeted research in the future focusing on specific microbial functions and the genes that control them — nitrogen fixation, for example — for a more in-depth understanding of the effects of temperature changes on each of the processes that may affect carbon cycling in the future. And Yuan added that it will be useful to continue monitoring the study site to get a sense of whether the microbial communities may change in different ways over a longer time span.
In general, focused research may help produce more precise ecosystem models that can better inform scientists about the effects we can expect from future warming in the Arctic, the paper’s authors point out. According to Zhou, current models include little information about the microbes. “If we really want to predict the future of the climate change effect on ecosystems, we need to consider microbial communities and their community structures,” he said.
But for now, breaking the ice on this field of study, so to speak, is an accomplishment in and of itself.
“This is a very good kind of breakthrough,” Romanovsky said. “People were already thinking about it, and this is actually showing that yes, changes in [physical] components of the system actually triggers changes in microbiology as well.”